A magnetic resonance (MR) examination system comprises a superconductive magnet with a cryogenic vessel, in which main magnet coils of the superconductive magnet are mounted. The cryogenic vessel typically comprises two vessels, which are mounted spaced apart to achieve thermal isolation, and a mounting structure, which is located within the two vessels, for mounting main magnet coils of the superconductive magnet. There are two ways to keep the main coils at superconducting temperature. In the first way the superconducting coils are in contact with liquid cryogen. In the second way the superconducting coils are directly cooled from a coldhead, e.g. via copper strands. In the first case, the mounting structure is typically provided as a third, inner vessel, which contains the cryogen, and the main coils are mounted inside the inner vessel. The cryogen is typically provided as liquid having a low boiling temperature, e.g. a boiling temperature of about 4.2 K in case of helium, which already evaporates when small amounts of heat enter.
FIG. 1 shows a state of the art superconductive magnet. The superconductive magnet 10 comprises a cryogenic vessel 12 and a set of main magnet coils 14, 16. The cryogenic vessel 12 comprises three vessels 22, 24, 26, which are mounted spaced apart to achieve thermal isolation. The three vessels 22, 24, 26 are an inner vessel 22, also referred to as 4K vessel when helium is used, a radiation shield 24, which is provided as a vessel surrounding the inner vessel 22, and an outer vessel 26, also referred to as 300K vessel, surrounding the radiation shield 24. The outer vessel 26 is usually made of stainless steel or aluminum and the radiation shield 24 of aluminum. The main magnet coils 14, 16 are mounted at an inner side of the inner vessel 22 along inner and outer cylindrical walls 28, 30 as inner and outer coils 14, 16, respectively. The inner vessel 22 contains a cryogen, e.g. liquid helium, which cools the main magnets 14, 16 and also enables heat buffering. Such a superconductive magnet is e.g. known from U.S. Pat. No. 7,170,377 B2.
Gradient switching induces dissipation in this superconductive magnet. This leads to boil-off of the cryogen in the cryogenic vessel. Former superconductive magnets simply blow the cryogen into the air when boil-off occurred. As a consequence cryogen had to be refilled frequently. State-of-the-art superconductive magnets have zero boil-off. Therefore, the dissipation in the magnet leads to an increase of pressure in the inner vessel. This phenomenon is called dynamic boil-off (DBO).
In detail, dynamic boil-off is caused by lack of eddy current shielding of the radiation shield due to mechanical resonances. When a gradient coil is switched, its stray field changes. As a consequence, eddy currents are induced in the radiation shield. Due to the eddy currents, forces are applied to this shield and the shield might start moving, e.g. oscillating. Its behavior depends on mass, stiffness and geometry of the radiation shield. Accordingly, the dynamic boil-off is a function of the frequency of the gradient switching. The DBO transfer function shows peaks as indicated in FIG. 2, which are related to mechanical resonances. This refers to mechanical resonances of the superconductive magnet, in particular of the radiation shield.
When the radiation shield would not move, which would imply infinite stiffness and infinite mechanical impedance, the attenuation of the stray field of the gradient coil would be monotonously increasing, i.e. only determined by the time constant of the shield. In that case, FIG. 2 would not show any peaks.
The peaks in the DBO graph of FIG. 2 are caused by mechanical resonances, as already discussed above. The mechanical impedance at a resonance is low. The lower the damping, the higher the Q factor and the lower the mechanical impedance. Since internal material damping decreases with lower temperatures, the magnitude of the damping can be several orders lower for very low temperatures close to OK, as indicated in FIG. 3. Hence, also the impedance of the radiation shield is low, so that it moves easily with the applied magnetic field and can enter in resonance.
FIG. 4 shows another state of the art superconductive magnet. The superconductive magnet 10 comprises in accordance with the typical cryogenic vessel 12 of FIG. 1 and a set of main magnet coils 14, 16. The cryogenic vessel 12 comprises two vessels 24, 26, which are mounted spaced apart to achieve thermal isolation. The two vessels 24, 26, refer to the radiation shield 24 and the outer vessel 26 as described above. The cryogenic vessel 12 further comprises a mounting structure 32, which is located inside the radiation shield 24. The main magnet coils 14, 16 are mounted to the mounting structure 32. Cold heads 15 are in contact with the main magnet coils 14, 16 to keep them at superconducting temperature. Accordingly, this superconductive magnet does not require the use of a cryogen.
The superconductive magnet without cryogen does not suffer from boil-off. Nevertheless, also for this type of superconducting magnet it is important to reduce heat from gradient switching, i.e. from movements of the radiation shield due to due to mechanical resonances, since there is no buffering/cooling from the cryogen for locally generated heat.
One approach is to reduce resonances in relevant frequency ranges based on geometry of the superconductive magnet and the cryogenic vessel. For example, thickness of the radiation shield can be increased to shift resonance frequencies into frequencies ranges, which are not relevant for the above problems like boil-off. As a consequence, superconductive magnet coils have a larger radius as well. This requires more superconductor material for the same magnetic field in an imaging volume of the MR examination system. A too thick radiation shield has the additional disadvantage that it might be destroyed due to the eddy current forces from a quench. A thinner radiation shield is also not an option. A too thin radiation shield does not conduct the heat sufficiently towards the cold head of the cryogenic system.
Another approach is based on material properties, e.g. on the choice of material. The resonance frequency is determined by E/ρ, where E is the Young's modulus E and ρ refers to the specific mass. The radiation shield is usually made from aluminum. E/ρ is relatively high for aluminum compared to other materials suitable for the use in cryogenic vessels for superconductive magnets. Hence, a change of material cannot reasonably reduce the above problems.
Another potential option that has proven not applicable is to add a visco-elastic layer to reduce the magnitude of the resonances of the cryogenic vessel. This is, however not feasible. First of all, the damping property of visco-elastic material is negligible at low temperature. Second, visco-elastic materials tend to generate gasses, which cause serious problems in a high-vacuum environment such as a superconducting magnet.
Apart from the cryogen problems described above in respect to the boil-off, there is also a risk of a magnet quench. As a consequence additional measures have been taken in the superconductive magnet, which are expensive and can add costs of several thousands of Euros to a single superconductive magnet.
The U.S. Pat. No. 6,038,867 shows a superconducting magnet with insulating blankets. The known superconducting magnet comprises a helium pressure vessel which contains a superconducting magnet coil assembly. The Helium pressure vessel is surrounded by a thermally insulating shield (between the He-vessel and a vacuum vessel. Additionally, thermal insulation blankets are disposed between the radiation shield and the vacuum vessel. Each thermal insulation blanket is formed as a plurality of thermally reflective (Al) sheets separated by low conductively metal spacer sheets.